Discover, Search Engine Watch Forums moderator, shared a screen capture to explain the extra step you need to take to ensure you actually exclude your ads from showing up on sites (AdSense publishers or parked domains) that you do not want them to show up on.

As you can see, first you type in the domain you want to block, then you click "Exclude Sites" and then one more step, you need to click, "Save All Exclusions."

Discovery explains:

Anyhow, I don’t know if this new process has resulted in many advertisers thinking they added new sites to their list but didn’t. If you have a list as large as ours its not very easy to see what has and has not been successfully added.